[refactor-scopes] Renamed RevealCK::Templates::RenderScope to RevealCK::Render::Scope
[refactor-scopes] Split contents_of implementation into a module
[refactor-scopes] Split css_files implementation into a module
[refactor-scopes] Enhanced Scope, replaced IndexHtmlRenderScope with it
Refactor. No functional code change.

Merge branch 'refactor-scopes' into upcoming

* refactor-scopes:
  [refactor-scopes] Enhanced Scope, replaced IndexHtmlRenderScope with it
  [refactor-scopes] Split css_files implementation into a module
  [refactor-scopes] Split contents_of implementation into a module
  [refactor-scopes] Renamed RevealCK::Templates::RenderScope to RevealCK::Render::Scope